In digital cable television systems, encryption is commonly used for blocking channels that have not been paid for. Those channels can generally only be accessed once an appropriate decryption key is available to the customer premise equipment (CPE; e.g., a television set-top box (STB), or properly enabled digital television set). Decryption keys are most commonly conveyed to the CPE using entitlement control messages (ECMs) that are transmitted from the network operator or an equivalent authority to the CPE. The content which is to be protected is most commonly encrypted using known algorithms such as the advanced encryption standard (AES), the DVB Common Scrambling Algorithm, the data encryption standard (DES) or its variants. Since these encryption/decryption algorithms themselves are publicly available, the key to the security of the content lies in the ability to conceal the decryption keys necessary to decrypt the content. In most subscription or pay-TV systems, the key-carrying ECMs are transmitted concurrently with the content in a single, common data transport stream.
For this reason, decryption keys transmitted via the ECM are commonly encrypted using proprietary encryption techniques. The encrypted keys are then commonly transmitted to the CPE via the ECM on an inband channel, changing on a very frequent basis in order to thwart a hacker's ability to decrypt the content based on an exhaustive trial and error key search. Unfortunately, one of the very tools used to thwart the hacker (frequent variation of the key and transmission of ECMs containing encrypted keys) provides a clue to the hacker to help crack the encryption algorithm. Each time a key is transmitted, more data is available to the hacker that can potentially be used to crack the encryption algorithm itself. Should the algorithm be cracked, the content is then unprotected, requiring costly revisions to the encryption algorithm and re-provisioning or even physical replacement of the CPE.